1. Field of the Invention
The present invention relates to a thin film forming apparatus, and more in particularly to a thin film forming apparatus in which a thin film is formed on a specimen put in a chamber having a window first by introducing electromagnetic waves such as rays of light, microwaves or the like into the chamber through a dielectric provided on the passing line of the electromagnetic waves, then activating the material gas in the chamber with the use of these electromagnetic waves.
2. Description of the Prior Art
As for apparatuses to form a thin film on a specimen, several ones can be enumerated, such as, for example, a photo CVD (Chemical Vapor Deposition) apparatus in which a thin film is formed on a specimen by introducing the rays of light into a chamber through a dielectric such as quartz glass or the like and activating the material gas in the chamber with the use of the rays of light, a microwave plasma CVD apparatus and an ECR (Electron Cyclotron Resonance) plasma CVD apparatus in which a thin film is formed on a specimen by introducing microwaves into a chamber through a dielectric and activating the material gas in the chamber with the use of microwaves, and a sputter apparatus wherein plasma caused by electromagnetic waves is utilized as the sputter source, for example, an ECR sputtering apparatus. In an ECR plasma CVD apparatus, the chamber is normally divided into a plasma-generating cell in order to generate plasma with the use of microwaves and a specimen-cell in order to form a thin film on the surface of the specimen with the use of plasma.
In a case where the rays of light or microwaves are introduced into the chamber through the dielectric in order to form a thin film on the specimen, a pile of films is generated on the inner surface of the dielectric to lower the transmissivity of the rays of light or the microwaves with respect to the dielectric. If such a state advances, there arises a trouble that the formation of the thin film on the surface of the specimen in the chamber cannot be performed sufficiently.
Therefore, in order to avoid such a state, the following preventive measures have conventionally been taken. For example, as shown in FIG. 1, a purge gas is blown against the inner surface of the dielectric 2 mounted on the chamber wall 1 in order to close the window 1a of the chamber C, from the gas introducing pipe 10 whose blowing outlet 10a is disposed near the window 1a in the chamber C, to check the generation of the pile of films. (See "Oyo Buturi" Vol. 55, No. 6 pp. 606-611.) Another measure is shown in FIG. 2 wherein a low vapor pressure substance 11 such as FOMBLIN OIL is applied on the inner surface of the dielectric 2 mounted on the chamber wall 1 in order to close the window 1a of the chamber C, to prevent the generation of the pile of films. (See "Solid-State Physics", Vol. 20, No. 8, pp. 564-566.) In both figures, the white arrow shows the introducing direction of the rays of light or microwaves.
However, it is not possible to prevent positively the generation of the pile of films only by blowing a purge gas against the inner surface of the dielectric, as shown in FIG. 1. In the case where a low vapor pressure substance 11 is applied on the inner surface of the dielectric 2, as shown in FIG. 2, there arises a problem that organic substances resulting from the substance 11 will mix in the thin film formed on the specimen.
When a film with a low photo-transmissivity with respect to the dielectric, such as amorphous silicon film, metallic films or the like is formed in the photo CVD, the transmissivity of the rays of light remarkably lowers, until the film-forming operations on the specimen come to stop as a result of the rays of light being reflected and absorbed by the film formed on the dielectric, even if the film-forming speed on the dielectric is reduced, according to the ways mentioned above. Also in the microwave plasma CVD as well as in the ECR plasma CVD, when a film with a low microwave-transmissivity with respect to the dielectric, such as a metallic film, Al, W, Mo or the like is formed, the microwave-transmissivity remarkably lowers, until the film-forming operations on the specimen come to stop as a result of the microwaves being reflected and absorbed by the film formed on the dielectric. Consequently in the conventional apparatuses as mentioned above it has been impossible to perform stable forming operations of a thin film.